Chemical projectile-target interaction and liquid immiscibility in impact glass from the Wabar craters, Saudi Arabia

详细信息    查看全文

• 作者：Christopher Hamann ; Lutz Hecht ; Matthias Ebert ; Richard Wirth
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2013
• 出版时间：15 November, 2013
• 年：2013
• 卷：121
• 期：Complete
• 页码：291-310
• 全文大小：3945 K

文摘

Impact glasses are usually strongly affected by secondary alteration and chemical weathering. Thus, in order to understand relevant formation processes, detailed petrographic studies on unweathered impact glasses are necessary as preserved heterogeneities in quenched impact glasses may serve as a tool to better understand their genesis. Here, we report on petrography and microchemistry of impact glasses from the Wabar impact craters (Saudi Arabia) that, with an age of ¡«300 years, are among the youngest terrestrial impact craters. The fact that parts of the IIIAB iron meteorite have survived impact and subsequent weathering is granting Wabar a special role among the presently 184 confirmed terrestrial impact structures. Electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) obtained on the black impact melt/glass variety at Wabar suggest that meteoritic Fe was selectively mixed with high-silica target melt at high temperatures due to selective oxidation, resulting in high Fe/Ni ratios for the black melt (37 on average, individual values range from 13 to 449) and low Fe/Ni ratios for projectile droplets (¡°FeNi spheres¡± with a Fe/Ni ratio of 3 on average; Fe/Ni ratio for the meteorite is ¡«12). The black melt shows emulsion textures that are the result of silicate liquid immiscibility. Liquid-liquid phase-separation resulted in the formation of a poorly polymerized, ultrabasic melt (Lfe) rich in divalent cations like Fe²⁺, Ca²⁺, or Mg²⁺, that is dispersed in a highly polymerized, high-silica melt (Lsi) matrix. The typical Wabar black melt emulsion displays a spheres-in-a-matrix texture of ¡«10-20 % Lfe homogeneously dispersed in the form of two sets of spheres and droplets (10-30 nm and 0.1-0.4 ¦Ìm in diameter) in ¡«80-90 % Lsi matrix, plus occasionally disseminated FeNi spheres. Around large (>10 ¦Ìm) FeNi spheres, however, the typical emulsion texture changes to ¡«21 % Lsi dispersed in ¡«79 % Lfe. This change of texture is interpreted as evidence for the transfer of meteoritic Fe from the meteoritic FeNi spheres into the target melt due to selective oxidation of Fe over Ni and Co. Variations in the bulk composition of Wabar black melt largely depend on the volume ratios between immiscible ultrabasic Lfe, felsic Lsi, and remains of meteoritic FeNi spheres. Based on natural occurrences of phase-separated glasses (this work and literature) and quenching experiments (literature), there is growing evidence that liquid immiscibility is a major process in the formation of glassy impactites.